This invention relates to a liquid crystal electro-optic device having good electrical characteristics and angle of visibility characteristics with which a uniform display can be obtained over an entire screen.
As a method of widening the angle of visibility of a liquid crystal electro-optic device, a method wherein the direction of an electric field impressed on a liquid crystal is made substantially parallel to the surface of a substrate (hereinafter referred to as the super TFT method) is disclosed for example in Japanese Unexamined Patent Publication No. H.6-160878. In this case, an electric field is induced between a source electrode and a common electrode formed on one substrate, and the liquid crystal molecules are oriented in the direction of this electric field. Also, in Japanese Unexamined Patent Publication No. H.6-214244, the electric field impressed on the liquid crystal is made uniform by making the height of the electrodes the same as the cell thickness.
In this kind of liquid crystal electro-optic device, because switching is carried out with the long axes of the liquid crystal molecules kept parallel with the substrate, there is no change with angle of visibility in the optical characteristics of the liquid crystal. Consequently, there is less light leakage and contrast reduction and the like resulting from angle of visibility than with conventional TN and STN methods.
However, electrodes of the super TFT method conventionally used have been of a trapezoidal or rectangular structure, and the electric fields produced by these electrodes have been noncontinuous at vertices of the trapezoid or rectangle. Consequently, the electric field impressed on the liquid crystal has changed at certain points. That is, the electric field (electric flux density) has changed suddenly at the vertices of the trapezoid or rectangle. Consequently, switching of the liquid crystal by the electric field has not been carried out evenly in the cell, and a phenomenon of the time taken for the electric field to change from OFF to ON or from ON to OFF (these are respectively called the rise time and the fall time) varying within the cell has appeared.
This is a shortcoming which appears particularly markedly in the super TFT method, wherein a horizontal electric field is used to carry out liquid crystal driving.
The above-mentioned electric field noncontinuity will be explained with reference to FIG. 1. Here, for simplicity, the state of lines of electric force around the electrodes when a voltage is impressed across a pair of parallel electrodes (101, 102) each of a rectangular cross-section of height ‘a’ and width c formed with a spacing 2b between the electrodes on an insulating substrate (103) will be described. (For lines of electric form formed by electric changes, please refer to works on electromagnetism, for example ‘Electromagnetism’, Kazukiyo Nagata, published by Asakura, or ‘Detailed Electromagnetic Practice’, Goto and Yamazaki, Kyoritsu publishing.) Here, a direction parallel with the substrate and perpendicular to (the height direction of) the electrodes will be made an x-axis and a direction perpendicular to the surface of the substrate will be made a y-axis. An origin will be so defined that the electrode surfaces parallel with the substrate are at y=0.
(1) In the region y<0 (−b≦x≦b), i.e. the region between the electrodes:
Because electric charge can be regarded as being distributed evenly over the electrode surfaces (104, 105), the lines of electric force (106) here are perpendicular to the electrodes (and parallel with the substrate).
(2) In the region y>0, i.e. the region above the electrodes:
Here, for the sake of simplicity, the state of the lines of electric force in the xy plane will be investigated.
Electric charge can be regarded as being distributed evenly over the electrode surfaces (107, 108).
For any point in the region y>0, the distance from the origin will be written r and the angle made by r and the x-axis will be written θ. Also, expressing z as a point in a complex plane using x, y and r, θ, the following relationship holds:z=x+iy=rexp(iθ)Here, to simplify the analysis, a value w will be defined as follows:w=A log z (A is a constant of proportionality). If the real and imaginary parts of w are written u and v, then:w=u+iv=A log z andu+iv=A log {rexp(iθ)}=A log r+iAθis obtained. Therefore,u=A log r,v=Aθ
Therefore, the set of curves expressed u=constant in the w plane are the set of curves r=constant in the xy plane, i.e. the set of concentric circles about the origin.
This result is illustrated in FIG. 1, from which it can be seen that the electric field distributions of the electrode side surfaces and the electrode top surfaces are different.
Here, as an example, the electric field between electrodes whose cross-sections are rectangular was shown, but the situation is the same between electrodes whose cross-sections are trapezoidal also. This is because since electric fields are formed perpendicular to the electrode surfaces the electric field of the taper parts and the electric field of the parts parallel with the substrate are noncontinuous at the electrode vertices.
This kind of noncontinuity of the electric field at the electrode vertices is a problem which cannot be ignored when making very small pixels. This is because when as a result of the adoption of very small pixels the number of electrodes increases and the interelectrode distance becomes small the noncontinuous electric field distributes at a high density.
As another method of solving the above-mentioned problem, an invention wherein in order to impress an electric field on the liquid crystal evenly in the cell thickness direction the height of the electrodes is made the same as the thickness of the cell has been proposed, in Japanese Unexamined Patent Publication No. H.6-214244. However, in making extremely tall electrodes, the following technological difficulties arise.
Firstly, when the height of an electrode is made as great as the cell thickness, a large difference in the horizontal direction electrode thickness tends to arise between the top and the base of the electrode. In the super TFT method, wherein the liquid crystal is driven with a horizontal electric field, a difference in the electrode thickness constitutes a difference in the interelectrode distance. Consequently, because the electric field strength in the cell thickness direction varies within the same pixel, driving the liquid crystal becomes difficult.
Secondly, when the electrodes are extremely tall, the coverage of layers formed on top of the electrodes is poor and line breakage tends to occur.
Thirdly, in making very small pixels, with extremely tall electrodes it is difficult to make the horizontal direction film thickness thin and obtain a large taper angle.
Consequently, in making very small pixels, to solve the above-mentioned problems, an electrode structure which can be made by a simple method and which also does not produce a noncontinuous electric field has been being sought.